ABSORPTION BY SOILS 6l 



solved substance in different degrees. Speaking generally, paper 

 absorbs dyes more readily than do soils, while soils absorb bases 

 more readily than does paper. Hence the reddening of litmus 

 paper when in contact with a moist soil. Heavy soils or soils 

 containing much hydrated ferric oxide absorb bases more readily 

 than do light soils, but this is probably owing to relative amounts 

 of surface exposed, for the same relation holds true with r"espect 

 to phosphoric acid. Soils rich in humus are better absorbers 

 than soils not so rich. But here again there is yet doubt as to 

 whether the explanation lies in the amount or in the kind of 

 surface acting. 



From the same solvent different dissolved substances are 

 absorbed quite differently by any given absorbent. This can be 

 readily illustrated again by dyes. If an aqueous solution of a 

 mixture of methylene blue and sodium cosine, for instance, be 

 shaken up with a soil, or percolated through a column of soil, 

 the methylene blue is absorbed the more quickly and completely 

 and a partial separation of the two dyes can be readily effected, 

 the separation being more or less complete according to the 

 conditions of the experiment. In the same manner two salts in 

 solution can be separated partially at least. 1 Soils absorb potas- 

 sium more readily than sodium; magnesium more readily than 

 lime; and ammonia more readily than any of these bases. 2 



The absorption from aqueous solutions of inorganic salts 

 involves a most interesting complication. Just as a mixture of 

 two or more dyes or salts in solution can be separated by the 

 selective action of an absorbent, so can an electrolyte itself be 

 decomposed or resolved. Thus, if a solution of potassium 



1 For a number of interesting examples, see, Ueber das Auf steigen 

 von Salzlosungen in Filtrirpapier, von Emil Fischer und Edward Schmid- 

 mer, Liebig's Annalen der Chemie, 272, 156-169 (1893). 



1 The prompt absorption of a base by soils is shown by the following 

 experiment : To some freshly boiled distilled water add several drops 

 of alcoholic phenolphthalein, and then just enough base to produce a 

 decided red color. If the solution be now passed through a short column 

 of soil, cotton, shredded filter-paper or similar absorbent, the percolate 

 will be perfectly colorless. The red color will be restored, however, by 

 adding a little of the base to the percolate. 



